8.8 Extreme Events

Since the SAR, there has been more attention paid to the analysis of extreme
events in climate models. Unfortunately, none of the major intercomparison projects
such as AMIP and CMIP have had diagnostic sub-projects that concentrated on
analyses of extreme events. Very few coupled models have been subjected to any
form of systematic extreme event analysis. Intercomparison of extreme events
between models is also made very difficult due to the lack of consistent methodologies
amongst the various analyses and also to the lack of access to high-frequency
(at least daily) model data. Analysis has also been limited by the comparatively
low resolution at which most models are run, this presents difficulties since
most extreme events are envisaged to occur at the regional scale and have comparatively
short lifetimes. However other forms of extreme event analysis have been developed
which use the large-scale fields produced by a climate model and produce various
indices of extreme events; such indices include maximum potential intensity
of tropical cyclones (Holland, 1997) or maps of 20-year return values of variables
such as precipitation or maximum temperature (Zwiers and Kharin, 1998) (a 20-year
return value implies that the value given is reached once in every 20 years).

In this chapter we assess the following types of extreme events that can be
presented in terms of global patterns; frequency of tropical cyclones, daily
maximum and minimum temperature, length of hot or cold spells, and precipitation
intensity and frequency (floods and droughts). While it is arguable that extra-tropical
cyclones belong to the class of "extreme events" we choose to include
them here for consistency with other chapters. Table 8.4
summarises the climate models and the types of extreme events that have been
analysed since the SAR. Assessments of extreme events that are purely local
or regional are discussed in Chapter 10.

Table 8.4: Analyses of extreme events in GCMs since
the SAR. Wherever possible the model names have been made consistent with
Table 8.1; however, since much of the analysis
has been done with AGCMs alone (and often with comparatively old model versions)
there often is no correspondence between these two tables. The references
given refer to the particular analysis used, and are not necessarily tied
to a specific model description.